Heretofore, it has already been known to a base member comprising a molded corrugated cardboard and a decorative skin layer laminated to the surface thereof as a base member for interior trim in automobiles, ships, houses, etc., for example as an interior trim base for the ceiling of an automobile.
In producing such base member for interior trim, it is necessary that corrugated cardboard be molded under heating and pressure into a roof shape of an automobile for example alone or together with a skin layer.
However, since corrugated cardboard itself does not have elasticity in the case where corrugated cardboard after molding has a portion of a large curvature such a deep-drawn portion, the inner surface of that portion is often wrinkled or broken.
Such wrinkle or rupture of the corrugated cardboard after molding appears as a scar on the surface of the skin layer, resulting in impairment of the external appearance and deterioration of the commercial value.
As a measure against such wrinkle or rupture of the corrugated cardboard after molding, there is described in Japanese Patent Publication No. 23534/80 a method wherein corrugated cardboard is subjected to drawing so that the clearance between upper and lower dies of a pressing die is smaller in the portion of the corrugated cardboard after molding where wrinkle or rupture is apt to occur than in the other portion, that is, the crushing thickness for such former portion is larger than that for the other portion.
However, the molded corrugated cardboard obtained by such method involves a problem in point of rigidity in some particular use because the core part of the swaged portion is crushed. Also in moldings it is necessary to use a special molding die wherein the clearance between upper and lower dies is smaller in the portion where wrinkle or rupture is apt to occur in the corrugated cardboard after molding than in the other portion, thus resulting in increase of the manufacturing cost.
In Japanese Patent Publication No. 55-34685 there is described a corrugated cardboard molding method wherein, in molding a corrugated cardboard base under heating and pressure, a notch is formed beforehand in the portion thereof to be subjected to drawing so that at the time of drawing the said notch is opened to permit elongation of the drawn portion of the corrugated cardboard and thereby prevent cracking of the same portion.
However, such conventional method premises forming a mounting hole for a functional component such as a room lamp in the drawn portion, and the aforementioned notch is formed in a narrower range than the hole opening range, then the notched portion is removed in the hole opening process, so that in the case where a hole is not formed in a trim panel, the notch appears as a scar on the skin layer surface. Therefore, such conventional method is not applicable.
Also known is a method wherein the liner of the portion where wrinkle or rupture is apt to occur in the shape after molding, such as as the inner surface of the portion to be subjected to drawing of corrugated cardboard, is slit in advance, followed by pressing, allowing the slit to accept elongation of the liner to thereby prevent the occurrence of wrinkle or rupture as far as possible. According to this conventional method, there is used a corrugated cardboard wherein a pair of liners and a core are bonded together with an adhesive, e.g. a thermoplastic resin, and at least one linear and the core are fixed to each other temporarily in a weak peeling strength, which corrugated cardboard is described in Japanese Patent Publication No. 56-30179, then the liner fixed temporarily to the core is once peeled off, then is slit using a slitter or the like and thereafter again laminated to the core. Thus, the operations are complicated and hence the productivity is poor. Besides, even by this conventional method, it is difficult to completely prevent the occurrence of wrinkle or rupture.